I want to know which of the groups will be given higher priority $\ce{-C#CH}$ or $\ce{-C(CH3)3}$ and why. I tried resolving the triple bond to single bonds such that they are phantom atoms. But that gave me a structure in which there are no atoms after the first point of difference. How is this (see image) wrong and if this is correct then isn't it a contradiction? After first point of difference, there are two difference one is nothing v/s $\ce H$ and other is $\ce C$ vs $\ce H$ and both situation seem to contradict in giving their respective group higher priority order.

enter image description here

  • $\begingroup$ I'll point out that there are different schemes for naming compounds. So the answer probably depends on exactly which one that you are using... $\endgroup$ – MaxW Jul 17 '18 at 15:27
  • $\begingroup$ @marxw for E and Z or R and S we need to first give them priority order. And i want to know which priority is correct and why. $\endgroup$ – Jasmine Jul 17 '18 at 16:12
  • $\begingroup$ At 2nd point of difference, the phantom carbon is attached to the original carbon and hence its a C-C v/s C-H and its obvious that the former will be given higher priority. Its not right to say that the phantom C is not linked to any atom if its really linked to the original C $\endgroup$ – Jasmine Jul 18 '18 at 18:27

From the Wikipedia article on CIP rules.

If an atom A is double-bonded to an atom B, A is treated as being singly bonded to two atoms: B and a "ghost atom" that is a duplicate of B (has the same atomic number) but is not attached to anything except A.

Ghost atoms are attached to resolve the triple/double bond.

So according to CIP rules the two compounds can be resolved as enter image description here
The pink atoms are ghost carbons

From here we can see that ethyne has a greater preference over isobutyl as after the first point of difference we move over to the next highest order carbon.In ethyne the second carbon is connected to $\ce{C2H}$ whereas the next highest order carbon in isobutyl is connected to $\ce{H3}$.

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